1. Field of the Invention
The present invention relates to a diagnostic system for an internal combustion engine. More specifically, the invention relates to a diagnostic system that is capable of determining whether a cooling water temperature sensor of an engine has failed.
2. Description of the Related Art
An engine temperature is a very important parameter that represents the condition of the engine. Especially, in an engine equipped with an electronic control system, the engine temperature is used for various controls. Usually, a cooling water temperature of the engine is used in lieu of the engine temperature. For example, in an engine equipped with an air-fuel ratio control feedback system, the feedback control of the air-fuel ratio is started when the engine cooling water temperature becomes higher than a predetermined temperature. Further, the fuel injection amount at the engine start is determined in accordance with the initial cooling water temperature, i.e., the cooling water temperature when the engine is started. Even after the engine has started, various corrections are made on the fuel injection amount based on the cooling water temperature.
Therefore, it is very important to ensure that the cooling water temperature sensor works correctly in order to perform a proper engine control. For this purpose, various diagnostic systems for detecting the failure of the cooling water temperature sensor have been proposed. For example, Japanese Unexamined Utility Model Publication (Kokai) No. 2-50043 discloses one type of such a diagnostic system.
The diagnostic system in the '043 publication uses a counter which measures the time elapsed after the engine starts. The system determines that the cooling water temperature sensor of the engine has failed if the cooling water temperature detected by the sensor does not reach a predetermined reference value after a certain fixed time has lapsed since the start of the engine.
The reference value is set at an actual cooling water temperature when the fixed time lapsed after the engine started, and is determined by an experiment in which the engine is operated in the conditions where the cooling water temperature rises most slowly, for example, in the condition where the ambient temperature is very low. Namely, the reference value is determined by experiment at a lowest possible cooling water temperature when the predetermined time lapsed after the engine starts with a consideration of a reasonable margin.
However, if the failure of the cooling water temperature sensor is determined based on the output of the sensor when a fixed time has lapsed after the engine started, as in the system of the '043 publication, an error in the diagnosis may occur. For example, in the '043 publication, the reference value used for determining the failure of the sensor is selected at a value which is a sum of the lowest possible cooling water temperature after the fixed time has lapsed and a reasonable margin. Therefore, the reference value is set at a considerably low temperature.
Since the reference value is very low in the '043 publication, if the cooling water temperature rises in a normal rate, the actual cooling water temperature becomes considerably higher than the reference value when the fixed time has lapsed. In this case, the cooling water temperature detected by the cooling water temperature sensor becomes higher than the reference value even if the output of the sensor is lower than the actual temperature to some extent and, thereby a failed sensor in which the output characteristics has changed may be determined as being normal.
This type of the error in the diagnosis may be avoided if the reference value is set at a high temperature so that the temperature detected by the failed sensor is always lower than the reference value. However, when the reference value is set at a high temperature, a long time is required before the actual cooling water temperature rises to the reference value especially when the rate of the temperature rise is low. Thus, in this case, the period from the engine start to the start of the diagnosis must be set long enough so that the actual cooling water temperature reaches the reference value even in the condition where the rate of the cooling water temperature rise is the lowest. This causes the time required for diagnosing the cooling water temperature sensor to be unnecessarily long in the most operating range of the engine.